Add gitsign to workspace static layers

[ChevronTango]

Is your feature request related to a problem? Please describe

Currently users wishing to make use of Gitsign must install it themselves in their workspace. This adds extra steps to the workspaces setup.

Describe the behaviour you'd like

All workspaces, regardless of the image being used, to have gitsign installed in the same way git and the gp CLI are, so that they can all make use of gitsign for signing commits.

Describe alternatives you've considered

Installing gitsign in individual workspaces would work, but wouldn't meet our goal of automatic signing of all commits without any user involvement. Adding additional layers to your workspace image would be "user involvement".

Additional context

This is part of the work in gitpod-io/gitpod#666 and follows on from gitpod-io/gitpod#17530

[ajhalili2006]

Could you please update the link for gitsign to https://docs.sigstore.dev/signing/gitsign/ since installation docs were moved without redirect? Thanks!